Hydrothermal Deposits - Metals and Society: An Introduction to Economic Geology - page 98

chemical sediments deposited from plumes of hydothermal fluid that precipitated
sulfides as they mixed with seawater after emission from vents on the sea floor. Other
researchers have argued for a syndiagenetic replacement origin for the ores - they
recognise that the banding is sedimentary in origin but argue that the original
sediments contained only barren Fe sulfides and silicates. According to them, the
ore metals replaced the original minerals as hydrothermal fluids circulated through
unconsolidated sediments tens or hundreds of metres below the sea floor. Although
the issue is not entirely resolved, it is very probable that both processes operated,
probably to different extents in different deposits. The Zn and Pb ores of the
McArthur River, for example, do seem to have formed as chemical sediments but
many aspects of Mt Isa ores point to replacement processes.
The processes implicated in the formation of a SEDEX deposit are illustrated in
Fig. 4.10 . The key is the deep circulation of fluids that are drawn down along the
margins of a sedimentary basin and pass through the sedimentary sequence before
being expelled on to the sea floor. The mineralizing episode is triggered by tectonic
events that activate major faults and generate rapid subsidence in the sedimentary
basin. The subsidence, perhaps aided by local heating from magmatic intrusions,
sets the circulating system into motion. Saline fluids become enriched in Fe, Zn, Pb
that are thought to be leached from iron oxides coating detrital sedimentary
minerals. The metals are transported in the hydrothermal fluids as chloride and
variable SO 4 2 complexes. When the fluids are discharged along faults to the basin
floor, metal sulphides precipitate at or above the seafloor by reaction with H 2 Sin
the overlying reduced anoxic layer at the base of the water column. The most likely
S source is biogenic H 2 S that is typically enriched in anoxic water columns.
Analysis: SEDEX Deposit
Source of metals - detrital sedimentary rocks
Source of S - biogenic H 2 S
Source of fluid - seawater and connate (interpore) water
Cause of fluid circulation - compaction(?), convection due to magmatic
intrusions
Precipitation process - cooling, reaction of oxidised fluid with H 2 Sin
anoxic seawater
4.3.4 Mississippi Valley Type (MVT) Deposits
This type of deposit is the antithesis of porphyry deposits: they form at very low
temperatures and they have nothing whatsoever to do with magmas. The name
comes from the valley of the Mississippi River in central USA where these deposits
were first recognized. They form a varied family of epigenetic lead-zinc ore
deposits that occur predominantly in carbonates of Paleozoic (Cambrian to